One of the major problems arising in the use of centrifugal vapor compressors for applications where the compressor load varies over a wide range is flow stabilization through the compressor. The compressor inlet, impeller and diffuser passages must be sized to provide for the maximum volumetric flow rate desired. However, when there is a low volumetric flow rate through such a compressor, the flow becomes unstable. Specifically, as the volumetric flow rate is decreased from a stable range, a range of slightly unstable flow is encountered. In this range, there appears to be a partial reversal of flow in the diffuser passage, creating noise and lowering the compressor efficiency. Below this range, the compressor enters what is known as surge, wherein there are periodic complete flow reversals in the diffuser passage, destroying the efficiency of the compressor and endangering the integrity of its elements.
Most centrifugal compressors employed in refrigeration systems are designed to rotate at a fixed operating speed. Capacity control of the machine is normally accomplished by varying the position of a series of adjustable guide vanes located at the inlet of the machine. The mass rate of flow of refrigerant delivered to the impeller is thus varied to meet the changing load demands made on the machine. At maximum flow, the refrigerant leaving the impeller is more than the diffuser can handle and the flow becomes choked at the diffuser throat. At lower flow rates, on the other hand, the flow of refrigerant moving through the diffuser becomes unstable and a partial flow reversal takes place producing noise and a dramatic reduction in machine efficiency. Eventually a complete reversal in flow is experienced whereupon the compressor stalls or surges. The range between a choke condition and the onset of a surge condition generally defines the operating range of the machine. In a compressor relying solely upon the inlet guide vanes for capacity control, this range is narrow, particularly when vanes are used in the diffuser.
Since a wide range of volumetric flow rates is desirable in many compressor applications, numerous modifications have been suggested to improve flow stability at low volumetric flow rates. As explained above, the prior art has attempted to control surge in centrifugal compressors by providing variable guide vanes that control the flow of refrigerant through the compressor. While this technique has helped, it has not been completely effective, as fluid flow through the diffuser accounts for most of the conditions that cause surge.
U.S. Pat. No. 5,807,071 issued to Brasz et al., which is incorporated herein by reference, discloses a variable geometry pipe diffuser for a centrifugal compressor that provides improved control of compressor performance under changing loads, to thus avoid instances of surge. The variable diffuser includes inner and outer rings having complementary inlet flow channel sections formed therein. The inner and outer rings are rotatable with respect to one another, thereby allowing for the adjustment of fluid flow through the flow channels.
While the variable diffuser disclosed in Brasz et al. provides a significant improvement in the art, it attempts to solve the surge problem independently of the variable guide vanes of the compressor. Specifically, the guide vanes and the diffuser are adjusted independently of one another, and this independent adjustment is incapable of maintaining maximum efficiency and avoiding surge completely. There still remains, therefore, a need for an efficient manner of controlling interaction between variable guide vanes and the variable diffuser of a centrifugal compressor in order to avoid instances of surge under changing load conditions.